In the Fiber-to-the-Home (FTTH) market, two technologies are competing: passive optical networks (PON) and Ethernet-based point-to-point networks. The European market is mostly directed to Ethernet-based point-to-point networks. The Japanese and to a lesser extent the US markets mainly works with PON. Ethernet point-to-point has considerable advantages over PON: Ethernet is an open standard, upgrading is easier and the network is future-proof. However, PON has a strong advantage as well: since PON networks are shared between end-users, the space and power consumption required for the central office is much smaller. In addition, the cost of the PON central office equipment is less than for point-to-point access switches. Reducing the size, power consumption and cost of Ethernet-based central office equipment (which consists of an individual transceiver per user) would therefore take away important drawbacks of point-to-point networks.
Photonic integrated circuits allow integrating multiple optical components on a single chip, thereby realizing the required reduction in size, cost and power consumption for the central office equipment. Contrary to traditional platforms for photonic integration such as III-V monolithic integration, which requires expensive regrowth techniques, or silica-on-silicon, which is not compatible with a compact form-factor, the silicon platform seems the only one which is truly compatible with large scale integration. This is due to the high refractive index contrast that is available in so called silicon photonic wires, allowing for wavelength scale routing and handling of light on the silicon-on-insulator chip. Wafer-level processing using CMOS fabrication tools and wafer level testing may enable low cost levels.
Using high index contrast photonic integrated circuits for communication applications however also bring along considerable challenges, especially regarding the efficient fiber-to-chip coupling and polarization independent operation of the photonic integrated circuit, and the need for handling widely spaced wavelength bands. Efficient optical coupling (−1.6 dB) between a single mode fiber and a silicon photonic wire has been demonstrated using a diffractive grating structure for a single polarization in the optical fiber. However, the 1 dB optical bandwidth is limited to 50 nm. In order to address the polarization sensitivity of the grating coupler, a polarization diversity approach has been presented using two-dimensional grating couplers to couple both orthogonal polarizations to the silicon chip. While this allows reducing the polarization dependent loss, it also only works over a limited wavelength range, which is insufficient for Fiber-to-the-Home (FTTH) transceiver applications.
In fibre-to-the-home (FTTH) optical networks, the upstream and downstream signals are typically sent over the same fiber, e.g. single mode fiber, but at a different wavelength. The upstream signals typically use a wavelength band around 1310 nm (1260 nm-1360 nm). The downstream signals typically use a wavelength band around 1490 nm (1480 nm-1500 nm). In FTTH transceiver applications, a 100 nm wide wavelength band needs to be covered in the 1310 nm wavelength range. Moreover, the two-dimensional grating approach introduces a decrease in fiber-to-chip coupling efficiency. In a FTTH transceiver at the end of the optical fibers, these 2 wavelengths need to be multiplexed and demultiplexed. The transceiver in the central office transmits 1490 nm and receives 1310 nm. The transceiver in the subscriber's home/building transmits 1310 nm and receives 1490 nm.
Sometimes the downstream signal also consists of 2 wavelength bands, one around 1490 nm and one around 1550 nm. This is illustrated in FIG. 1, indicating the upstream optical signal 2 being in a first wavelength range and the downstream optical signals 4 being in a second wavelength range comprising two downstream optical signals 6 in sub-wavelength ranges. In that case, these two downstream signals also need (de)multiplexing in the transceiver. Such (de)multiplexing may be implemented in a silicon-on-insulator photonic integrated circuit.
Since the FTTH transceivers require photodetection and light emission on the photonic integrated circuit, other materials than silicon need to be integrated in order to perform these functions. Ge-based photodetectors are a good option given their direct compatibility with the CMOS compatible processing of the silicon photonic integrated circuit. Another approach is using a heterogeneously integrated III-V semiconductor photodetector. This approach has the advantage that it also allows the integration of III-V light sources and III-V modulators on the same photonic integrated circuit.
In these approaches the 1310 nm band needs to be coupled into a waveguide, guided and detected in a highly efficient, polarization independent way and over a large wavelength span (e.g. 100 nm). In addition, the 1490 nm and 1550 nm light needs to be coupled out and demultiplexed from the incoming light. This approach makes it hard to reach the required specification for FTTH transceivers.